But I thought the Wikipedia article made it clear why space-time can be seen to function as a fourth dimension when relativity theory is applied. The actual curvature of spacetime, and time dilation effects, have been observed. Space and time are intricately connected.

Consequently, it is simple to read off the curvature of space from the cosmic microwave background: We know the absolute size of the largest sound waves in the early universe, and we can measure their apparent size in the sky. The distance of the microwave background can be calculated: We know the temperature at which it was formed, and we can measure its temperature today. The temperature difference is directly related to the amount by which the universe has expanded from then until now, and this, in turn, is directly related to the distance. Comparing the apparent and true sizes at the calculated distance, we obtain the curvature of space. In this way, astronomers have determined that, to high precision, space in our cosmos is flat.

The Maxima experiment, based at Berkeley, used a related balloon-borne system. The Microwave Anistropy Telescope, which made its own measurements from a high plateau in Chile, used an entirely different detector based on powerful electronic amplifiers.

All the data are consistent with a flat universe, said scientists on the projects and others who have read the teams' reports.

''These really favor a flat universe,'' said Dr. Alessandro Melchiorri, an astrophysicist at La Sapienza.